1. Field
The present invention relates to improvements to the surface picking techniques used in seismic processing.
2. Description of the Related Art
In the field of earth sciences, geophysicists use “seismic reflection” techniques in order to image the structure of the subsoil.
These consist of emitting acoustic signals at the surface of the ground, and in recording them after 10 their successive reflections at the boundaries of the geological strata.
These strata constitute the subsoil, a geometrical model of which is intended to be created.
One of the methods most commonly used in order 15 to do this, especially when prospecting for oil deposits, is the “3D acquisition” method.
This consists of distributing emitters and receivers in a horizontal surface plane along a grid of coordinates X, Y.
The result of such a method is a three-dimensional image of the subsoil, organized the form of recordings sampled as a function of time or depth, denoted Z.
Each of these recordings, which is referred to 25 as a “trace”, is a representation of the vertical in line with a point Pij with the coordinates Xi, Yj.
Such three-dimensional acquisition makes it possible to obtain a volume, the dimensions of which are two geometrical axes X, Y and one axis for the time or the acquisition depth Z.
In addition to these spatial dimensions (X, Y and the vertical axis Z), many other quantities are of interest to physicists, in particular:
The source/receiver distance or the angle of incidence which was used to generate the volume. Until recently, this distance (offset) was most often reduced to 0 and the volumes were obtained by adding contributions with a constant distance, so as to increase the signal/noise ratio. Partial summation by classes of offset or angle now allows geophysicists to carry out analyses by class of distance or angle.
Time. A plurality of seismic acquisitions can be carried out successively in time with the same network of receivers/emitters; comparison of the acquired data makes it possible, for example, to understand the variation of a fluid regime in the case of reservoir exploitation.
The type of seismic-waves.—The sensors can simultaneously record compression waves (P waves) and shear waves (S waves); the ratio between the arrival times of these two types of waves is a direct link with the nature of the medium through which the waves are propagating.
Velocity. Time or depth migration consists of shifting the points of the acquisition volume while taking a velocity field into account. The velocity used in these processes may be perturbed and constitute an extra analysis dimension. In order to update a velocity model, for example, the Applicant has proposed an approach in which perturbation factors are regularly distributed over a velocity model and, for a position X, Y, the value of the sample associated with the same reflector in each of the migrated cubes is addressed.
etc.
It is of fundamental importance to take these other quantities into account in many fields of geoscience, from velocity analysis to reservoir characterization.
This is now feasible by virtue of the increase in computer processing power and storage capacities; analysis and interpretation tools suitable for this new approach have yet to be provided.
When a quantity in addition to the three spatial dimensions, in which seismic volumes are generally represented, is currently taken into account with existing tools, the interpretation of the data is equivalent to the sum of the interpretations carried out independently on each of the 3D seismic volumes obtained for different values of said quantity.
Document WO 99/67660 describes a method for monitoring the variations of the subsoil. This method consists of determining a displacement field with which it is possible to convert from a sampled volume of seismic data, measured at a time T, to a field of seismic data measured at a subsequent time T, to a field of seismic data measured at a subsequent time T. The displacement field determined in this way is applied to the first seismic volume, so as to obtain an image of the subsoil.
The information which can be derived from such a way of interpretation is necessarily limited.
The invention itself provides a effective and original contribution to the resolution of this problem.
One technique conventionally used for the analysis of 3D seismic volumes is that of surface picking, an example of which is given in Patent FR 2 657 695. That document relates to a method for surface picking in a three-dimensional volume, according to which a volume representing the values of a sampled parameter is transformed into a two-dimensional image by using a Peano-Hilbert curve, and a automatic picking program is applied to the 2D image which is obtained.
When an extra parameter is taken into account in addition to the spatial dimensions which constitute the 3D volumes (X, Y and vertical axis) and said quantity is varied, one difficulty is due to the fact that the nature and the shape of the reflector to be interpreted may change from one volume to another, along the dimension constituted by said quantity.
Coherence between the various 3D volumes is not necessarily inherent in the mapped sample, which leads to selection problems in respect of the events interpreted by geophysicists. This may lead to discontinuities or aberrations in the surface picking.